Ferrite powder for bonded magnets, method for producing the same and ferrite bonded magnet

ABSTRACT

A ferrite powder for bonded magnets capable of producing a ferrite bonded magnet having high BHmax, and excellent in fluidity when converted to a compound, and having a high p-iHc value, and a method for producing the same, and a ferrite bonded magnet using the ferrite powder for bonded magnets, wherein an average particle size of particles obtained by a dry laser diffraction measurement is 5 μm or less; a specific surface area is 1.90 m2/g or more and less than 2.80 m2/g; a compression density is 3.50 g/cm3 or more and less than 3.78 g/cm3, and a compressed molding has a coercive force of 2300 Oe or more and less than 2800 Oe.

TECHNICAL FIELD

The present invention relates to a ferrite powder for bonded magnetsused for producing a bonded magnet and a method for producing the same,and a ferrite bonded magnet using the same.

DESCRIPTION OF RELATED ART

Ferrite sintered magnet (referred to as a “sintered magnet” in somecases in the present invention) is used for a magnet in which a highmagnetic force is required. However, the sintered magnet involvesinherent problems such that chipping or cracking occurs, with poorproductivity because polishing is required, and processing into acomplex shape is difficult. In recent years, there is a demand forreplacing the sintered magnet with the ferrite bonded magnet (referredto as a “bonded magnet” in some cases in the present invention).However, when compared to the sintered magnet, a maximum energy product(BH_(max)) is poor in the bonded magnet, and therefore in order toreplace the sintered magnet with the bonded magnet, property of B_(max)is requested to be improved in the bonded magnet.

Generally, BH_(max) is determined by a residual magnetic flux density(Br) and a coercive force (Hc).

Here, Br is expressed by the following formula 1, in which density ofmagnet is indicated by (ρ), a saturation magnetization of a magneticpowder is indicated by (σs), and an orientation is indicated by(Br/4πls).Br=4ζ×ρ×σs×(orientation)  Formula 1On the other hand, Hc is described by the theory of crystal anisotropy,shape anisotropy, and a single magnetic domain structure.

A typical difference between the sintered magnet and the bonded magnetis the value of ρ. ρ of the sintered magnet is about 5.0 g/cm³. Incontrast, it is a matter of course that density of the bonded magnet islower than 5.0 g/cm³ because a binder such as resin and rubber is mixedinto a kneaded material (compound) of a raw material in addition to aferrite powder. Therefore, Br of the bonded magnet is decreased.Accordingly, in order to increase a magnetic force of the bonded magnet,the content (F.C.) of the ferrite powder in the compound is required tobe increased.

However, when F.C. of the ferrite powder in the compound is increased,the viscosity of the compound becomes high during kneading of theferrite powder and the binder, thus increasing a load and reducing theproductivity of the compound, resulting in being unable to knead theferrite powder and the compound in an extreme case. Then, even if thekneading can be performed, the value of fluidity (MFR) is low at thetime of molding of the compound, thus reducing the productivity of themolding product, resulting in being unable to mold the compound in anextreme case.

In order to solve such a problem specific to the bonded magnet, thebonded magnet has been improved from a viewpoint of selection of thebinder and surface treatment of the ferrite powder, etc. However, it ismost important to guarantee high F.C. of the ferrite powder itself. F.C.of the ferrite powder is highly relevant to a particle size distributionand a compression density of ferrite particles constituting the ferritepowder.

As a method for producing such a ferrite powder for bonded magnets(referred to as “ferrite powder” in some cases in the presentinvention), an applicant discloses patent document 1.

In patent document 1, the applicant discloses the ferrite powderobtained by mixing the ferrite powder constituted of particles having aplurality of particle sizes. Therefore, in this ferrite powder, thereare a plurality of peaks in a particle size distribution.

Further, when the value of a specific surface are (SSA) is high in theparticles constituting the ferrite powder, resin (binder) amountadsorbed on the surfaces of the ferrite particles during kneading andmolding, is increased, resulting in a reduction of the ratio of theresin that can be freely moved, and causing a reduction of fluidity, andthe reducing of the fluidity further causing reduction of orientationduring magnetic molding, namely, leading to reduction of Br. Therefore,SSA is set to 1.8 m²/g or less.

On the other hand, 2100 Oe or more coercive force (p-iHc) of apressurized powder body (called “a compressed molding” hereafter) isrealized.

Further, 55 emu/g or more saturation magnetization value σs in anon-oriented state is realized.

PRIOR ART DOCUMENT Patent Document

-   Patent document 1: Japanese Patent Laid Open. Publication    No,2010-263201

SUMMARY OF THE INVENTION Problem to be Solved by the Invention

According to patent document 1 disclosed by inventors of the presentinvention, ferrite powder with high compression density and high fillingproperty can be obtained.

However, in recent years, a bonded magnet having a higher magnetic forceis demanded in the market. As a result, the ferrite powder having higherp-iHc is demanded.

However, in the present situation, when the F.C. value in the compoundis increased, dispersibility and MFR value of the ferrite particles aredecreased, thus making it difficult to mold a molding product, andmaking it difficult to knead a pellet.

Therefore, not a bonded magnet but a sintered magnet is used in a fieldin which a high magnetic force of about BH_(max)=2.5 to 4.0 MGOe isrequired. However, as described above, the sintered magnet involves aspecific problem of generating chipping or cracking and requiringpolishing, and therefore productivity is poor and processing into acomplex shape is difficult.

On the other hand, in recent years, bonded magnets using rare earthmagnets are used partially in this field. However, the rare earth magnetinvolves a problem that it is 20 times the cost of the ferrite magnet,and is easily rust.

Under such a circumstance, in the bonded magnet having good workabilityat a low cost, it is strongly requested to achieve higher BH_(max) inapplications of AV, OA equipment, small motors such as electricalcomponents of automobiles and magnet rolls of copying machines.

In view of such a circumstance, the present invention is provided, andan object of the present invention is to provide a ferrite powder forbonded magnets capable of producing a ferrite bonded magnet having highBH_(max), and excellent in fluidity when converted to a compound, andhaving a high p-iHc value, and a method for producing the same, and aferrite bonded magnet using the ferrite powder for bonded magnets.

Means for Solving the Problem

In order to solve the above-described problem, strenuous efforts aremade by inventors of the present invention, and it is found that in aproducing step of the ferrite powder for bonded magnets, a mechanicalpulverizing force is added on the ferrite powder in the stage of theproducing step, and the values of an average particle size, a specificsurface area (SSA), and a compression density (CD) of the producedferrite powder for bonded magnets are set in a prescribed range, tothereby obtain the ferrite powder capable of producing the bonded magnethaving excellent fluidity even if the F.C. value in the compound isincreased, and having a value suitable for its application. Thus, thepresent invention is completed.

Namely, a first invention for solving the abovementioned problem is aferrite powder for bonded magnets, wherein

an average particle size of particles obtained by a dry laserdiffraction measurement is 5 μm or less;

a specific surface area is 1.90 m²/g or more and less than 2.80 m²/g;

a compression density is 3.50 g/cm³ or more and less than 3.78 g/cm³,and

a compressed molding has a coercive force of 2300 Oe or more and lessthan 2800 Oe.

A second invention is the ferrite powder for bonded magnets, wherein aparticle size distribution curve has two mountains, and out of these twomountains, a particle size of a smaller particle size peak is 1.2 μm orless.

A third invention is the ferrite powder for bonded magnets, wherein acumulative distribution value at a particle size of 0.62 μm in acumulative particle size distribution curve, is 13 volume % or more.

A fourth invention is the ferrite powder for bonded magnets, wherein acumulative distribution value at a particle size of 0.74 μm in acumulative particle size distribution curve, is 17 volume % or more.

A fifth invention is the ferrite powder for bonded magnets, wherein afrequency distribution value at a particle size of 0.28 μm in afrequency distribution curve, is 6.0 or more.

A sixth invention is the ferrite powder for bonded magnets, wherein afrequency distribution value at a particle size of 0.33 μm in afrequency distribution curve, is 7.0 or more.

A seventh invention is the ferrite powder for bonded magnets accordingto any one of the first to sixth inventions, wherein when kneaded withnylon resin powder to obtain a compound with 92.7 mass % content of theferrite powder, fluidity of the compound is 55 g/10 min or more.

An eighth invention is a ferrite bonded magnet produced by molding theferrite powder for bonded magnets of any one of the first to seventhinventions.

A ninth invention is the ferrite bonded magnet containing the ferritepowder for bonded magnets of any one of the first to seventh inventions.

A tenth invention is a method for producing a ferrite powder for bondedmagnets, including:

granulating a plurality of ferrite raw materials including iron oxide,and obtaining a first granulated material;

sintering the obtained first granulated material at a first temperature,and obtaining a coarse powder of a sintered product;

granulating a plurality of ferrite raw materials including iron oxide,and obtaining a second granulated material;

sintering the obtained second granulated material at a secondtemperature lower than the first temperature, and obtaining fine powderof the sintered product;

mixing the obtained coarse powder and fine powder, to obtain a mixedpowder; and

adding a mechanical pulverizing force to the obtained mixed powder toobtain a pulverized mixture, and applying annealing to the obtainedpulverized mixture,

wherein the first temperature is 1220° C. or more and less than 1250°C., and the second temperature is 900° C. or more and 1000° C. or less,and when a mixing ratio of the coarse powder and the fine powder isexpressed by [mass of the coarse powder/mass of (coarse powder+finepowder)]×100%, it is 56 mass % or more and less than 75 mass %,

wherein, the mechanical pulverizing force is the force caused by apulverization processing in which a vibration ball mill with a capacityof 2 to 4 L and power of 0.3 to 0.5 kW, is loaded with steel ballshaving a diameter of 8 to 14 mm as media, so that pulverization isperformed at a rotation number of 1700 to 1900 rpm and an amplitude of 7to 9 mm, for 20 to 100 minutes as a processing time, or the forceequivalent thereto.

Advantage of the Invention

According to the ferrite powder for bonded magnets of the presentinvention, a ferrite bonded magnet having excellent fluidity even ifF.C. value in a compound is increased, and having high BH_(max) can beproduced, and the bonded magnet having a p-iHc value suitable for itsapplication can be produced.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a graph showing a particle diameter distribution curve of aferrite powder according to the present invention, wherein the verticalaxis indicates a frequency distribution and the horizontal axisindicates a particle size (μm).

FIG. 2 is a graph showing a cumulative particle size distribution curveof the ferrite powder according to the present invention, wherein thevertical axis indicates a cumulative particle size distribution (volume%) and the horizontal axis indicates the particle size (μm).

DETAILED DESCRIPTION OF THE INVENTION

After extensive studies by inventors of the present invention regardinga configuration realizing a bonded magnet having a high magnetic force,a ferrite powder for bonded magnets is achieved, in which an averageparticle size obtained by a dry-type laser diffraction measurement is 5μm or less, SSA is 1.90 m²/g or more and less than 2.80 m²/g, CD is 3.50g/cm³ or more and less than 3.78 g/cm³, and p-iHc is 2300 Oe or more andless than 2800Oe.

Then, it is found by the inventors of the present invention that highMFR value of 65 g/10 min or more can be guaranteed even at a high F.C.value such as F.C. of 92.7 mass %, and the bonded magnet having a highmagnetic force can be easily produced, by a compound obtained bykneading and mixing the ferrite powder for bonded magnets of the presentinvention having the above configuration, and resin.

Further, after studies on a method for producing the ferrite powder forbonded magnets according to the present invention having the aboveconfiguration, it is found that by adding a mechanical pulverizing forceto a powder in a stage of a producing step, thereby setting an averageparticle size, SSA, and CD of the produced ferrite powder for bondedmagnets to values in a prescribed range, to thereby obtain the ferritepowder having p-iHc suitable for the purpose of use, in which molding amolding product is easy, and kneading of a pellet is easy. Thus, thepresent invention is completed.

The present invention will be described hereafter in an order of 1.)Ferrite powder of the present invention, 2.) Method for producing theferrite powder of the present invention, 3.) Compound filled with theferrite powder of the present invention, and 4.) Bonded magnet with thecompound of the present invention molded.

-   1.) Ferrite Powder of the Present Invention

As described above, the ferrite powder of the present invention has aconfiguration in which an average particle size obtained by a dry-typelaser diffraction measurement is 1 μm or more and 5 μm or less, SSA is1.90 ²/g or more and less than 2.80 m²/g, CD is 3.50 g/cm³ or more andless than 3.78 g/cm³, and p-iHc is 2300 Oe or more and less than 2800Oe.

Wherein each configuration and an effect thereof of (1) Particle size,(2) SSA, (3) CD, (4) p-iHc, and (5) Residual magnetic flux density of acompressed molding in the ferrite powder, will be described.

-   (1) Particle Size

According to the ferrite powder of the present invention, an averageparticle size of particles is 5 μm or less and 1 μm or more obtained bythe dry-type laser diffraction measurement. This is because when theaverage particle size is 5 μm or less, an orientation and a coerciveforce after bond magnetized, can be guaranteed. Further, this is becausewhen the average particle size is 1 μm or more, a CD value can beguaranteed.

The particle size distribution curve of the ferrite powder of thepresent invention has two peaks, and out of these peaks, a smaller peakindicating a small particle size is preferably 1.2 μm or less. Bysatisfying this configuration, fluidity (MFR) during production andmolding of the compound described later can be increased. This isbecause when the size of the particles constituting fine powder is smalland even primary particles are dispersed, the fine particles can bedispersed between coarse particles. On the other hand, although the sizeof the primary particle can be made small by decreasing a sinteringtemperature for sintering the fine powder, it is difficult to dispersethe primary particles. Therefore, by increasing the pulverization timeby the dry-type pulverization method, even the primary particles withreduced particle size can be pulverized.

Incidentally, in the present invention, the peak does not necessarilymean that a maximum value of the peak is completely independent. Thatis, when a shoulder exists at a skirt portion of the peak, the shoulderis considered as another peak.

In a cumulative particle size distribution curve of the ferrite powderof the present invention, preferably a cumulative distribution value ata particle size 0.62 μm is 13 volume % or more, and a cumulativedistribution value at a particle size 0.74 μm is 17 volume % or more.

In other words, preferably a frequency distribution at a particle size0.28 μm is 6.0 or more and a frequency distribution at a particle size0.33 μm is 7.0 or more in the particle size distribution curve.

This is because when ferrite and resin are mixed and kneaded, gapsbetween (coarse) ferrite particles can be filled with not only resin butalso in a mixed state of resin and fine ferrite particles. As a result,high fluidity can be obtained even when a filling ratio of the ferriteis increased.

-   (2) Specific Surface Area (SSA)

It is found by the inventors of the present invention that SSA ispreferably 1.90 m²/g or more. Probably this is because mono-dispersionof particles constituting the ferrite powder is preferable fororientation, wherein the ferrite particles are mono-dispersed particlesbecause SSA is 1.90 m²/g or more.

On the other hand, since SSA is less than 2.80 m²/g, the followingsituation can be prevented: resin (binder) amount adsorbed on surfaceson the particles constituting the ferrite powder during kneading andmolding, is increased and the ratio of resin that can be freely moved isdecreased accordingly, thus causing a reduction of fluidity. Thistendency is remarkable when F.C. of the ferrite powder in the compoundis high, and a magnetic field orientation during formation of themagnetic filed is low, and therefore it is found that SSA is preferablyset to be less than 2.80 m²/g.

As described above, the inventors of the present invention achieve theconfiguration in which SSA is 1.90 m²/g or more and less than 2.80 m²/gin the ferrite powder of the present invention.

-   (3) Compressed Density (CD)

CD is an index indicating how much filling of the ferrite particles ispossible in a limited volume, the ferrite particles being a minimumconstituent unit of the bonded magnet, and has a high correlation with asaturation magnetic flux density (Bs). On the other hand, in the case ofa high CD, the volume of the gaps between particles becomes small, andtherefore a resin amount that enters into the gaps of the ferrite powderis seemingly decreased in the compound obtained by mixing and kneadingthe ferrite powder and nylon-6 resin for example. Therefore, it is foundthat CD is preferably set to 3.50 g/cm³ or more.

On the other hand, from a viewpoint of guaranteeing a high coerciveforce (inj-iHc) value of the bonded magnet (molding product) produced ina subsequent step, CD is preferably 3.78 g/cm³ or less.

As described above, the inventors of the present invention realize theconfiguration in which CD is 3.50 g/cm³ or more and less than 3.78 g/cm³in the ferrite powder for bonded magnets of the present invention.

-   (4) Coercive Force (p-iHc) of a Compressed Molding

p-iHc is the coercive force of the ferrite powder for bonded magnets, ina state in which there is a history of a mechanical stress caused bybeing compressed under a high pressure of 2 tons/cm². “Ton” is themeaning of 1000 kg. Generally, during kneading and molding for producingthe bonded magnet, the ferrite powder for bonded magnets is subjected tothe mechanical stress, and therefore the coercive force is lower thanthat in a powder state to which the stress is not added.

Here, since there is a high correlation between p-iHc and the coerciveforce (inj-iHc) of the bonded magnet (molding product), the p-iHc valueis an effective index for estimating the inj-iHc value. Accordingly,higher p-iHc is considered to be preferable, and from a viewpoint ofpreviously considering low temperature demagnetization, it is found thata preferable configuration is that p-iHc is 2300Oe or more.

On the other hand, in order to produce the bonded magnet, p-iHc ispreferably less than 2800 Oe from a viewpoint of guaranteeing easinessof magnetization when the molded molding article is magnetized.

As described above, the inventors of the present invention realize theconfiguration in which p-iHc of the compressed molding using the ferritepowder for bonded magnets of the present invention, is 2300 Oe or moreand less than 2800 Oe.

-   (5) Residual Magnetic Flux Density of the Compressed Molding (p-Br)

p-Br is a residual magnetic flux density of the ferrite powder forbonded magnets in a state in which there is a history of a mechanicalstress caused by being compressed under a high pressure of 2 tons/cm².

As described above, the ferrite powder of the present invention has theconfiguration in which the average particle size obtained by the drylaser diffraction measurement is 5 μm or less, SSA is 1.90 m²/g or moreand less than 2.80 m²/g, CD is 3.50 g/cm³ or more and less than 3.78g/cm³, and p-iHc is 2300 Oe or more and less than 2800 Oe. With thisconfiguration, by forming the ferrite powder, the bonded magnet having ahigh magnetic force of about BH_(max)=2.4 to 3.0 MGOe can be obtained.

There is a close interaction effects between SSA showing a surfaceproperty of each particle of the ferrite particles, and CD as a powderproperty of the ferrite powder for bonded magnets. Also, there is aninfluence on the magnetism depending on the orientation and a dispersionstate of the ferrite particles, and therefore there is a closecorrelation between SSA, CD, and p-iHc, and it is important to satisfythe abovementioned configuration.

In the abovementioned configuration, since SSA of the ferrite particlesis 1.9 m²/g or more and less than 2.8 m²/g, CD can be set to 3.5 g/cm³or more and less than 3.78 g/cm³ in the ferrite powder for bondedmagnets, and it is possible to obtain the bonded magnet which shows ahigh magnetic force of about BH_(max)=2.4 to 3.0 MGOe and high p-iHc ofthe ferrite particles in combination.

On the other hand, when p-iHc is 2300 Oe or more and less than 2800 Oe,the bonded magnet having BH_(max) =2.4 to 3.0 MGOe can be obtained. Suchp-iHc is also formed by the orientation of the ferrite particles underan action of SSA and CD of the ferrite particles. In this case as well,there is a close correlation between SSA and CD.

-   2.) Method for Producing the Ferrite Powder of the Present Invention

A method for producing a ferrite powder for bonded magnets according tothe present invention includes:

(1) granulating a plurality of ferrite raw materials including ironoxide, and obtaining a first granulated material;

(2) sintering the obtained first granulated material at a firsttemperature, and obtaining a coarse powder of a sintered product;

(3) granulating a plurality of ferrite raw materials including ironoxide, and obtaining a second granulated material;

(4) sintering the obtained second granulated material at a secondtemperature lower than the first temperature, and obtaining fine powderof the sintered product;

(5) mixing the obtained coarse powder and fine powder, to obtain a mixedpowder; and

(6) adding a mechanical pulverizing force to the obtained mixed powderto obtain a pulverized mixture, and applying annealing to the obtainedpulverized mixture.

-   (1) The Step of Granulating a Plurality of Ferrite Raw Materials    Including Iron Oxide, and Obtaining a First Granulated Material

Iron oxide and strontium carbonate are weighed in a molar ratio of ironoxide:strontium carbonate=5.50 to 6.00:1.

0.10 to 3.0 mass % of flux (oxide, inorganic acid, or salt thereof) isadded to the weighed material, and 1.00 to 5.00 mass % of chloride isadded and mixed thereinto, to obtain a mixture. The mixture isgranulated to obtain a particle with a dimeter of about 3 to 10 mm, tothereby obtain the first granulated material.

Here, bismuth oxide, boric acid, borates, sulfates, phosphates, silica,and silicates, etc., can be preferably given as oxides, inorganic acids,or the salt thereof, and two kinds Or more of them can be used incombination. KCl, NaCl, LiCl, RbCl, CsCl, BaCl₂, SrCl₂, CaCl₂, andMgCl₂., etc. can be preferably given as chlorides, and two kinds or moreof them can be used in combination.

-   (2) The Step of Sintering the Obtained First Granulated Material at    a First Temperature, and Obtaining a Coarse Powder of a Sintered    Product

The obtained first granulated material is sintered for 10 to 120 minutesat 1250 to 1290° C. under flowing atmosphere of the air, to therebyobtain a sintered product. Then pulverization processing is applied tothe sintered product using a roller mill or a hammer mill, to therebyobtain a raw material coarse powder of coarse particles.

-   (3) The Step of Granulating a Plurality of Ferrite Raw Materials    Including Iron Oxide, and Obtaining a Second Granulated Material

Iron oxide and strontium carbonate are weighed and mixed in a molarratio of iron oxide:strontium carbonate=5.20 to 6.00:1. After mixture,pulverization is performed to obtain particles having a diameter ofabout 3 to 10 mm.

-   (4) The Step of Sintering the Obtained Second Granulated Material at    a Second Temperature Lower than the First Temperature, and Obtaining    Fine Powder of the Sintered Product

The obtained second granulated material is sintered for 10 to 120minutes at 900° C. to 1000° C. which is a lower temperature than thefirst temperature under flowing atmosphere of air, to thereby obtain asintered product. The sintered product is pulverized by the hammer mill,to thereby obtain a raw material coarse powder of fine particles.

-   (5) The Step of Mixing the Obtained Coarse Powder and Fine Powder

The raw material coarse powder of coarse particles (70 to 80 pts.wt.)obtained by the abovementioned (2), and the raw material coarse powderof fine particles (35 to 15 pts.wt.) obtained by the abovementioned (4)are weighed (at this time, the sum of the raw material coarse powder ofcoarse particles and the raw material coarse powder of fine particles is100 pts.wt.). The obtained weighed product is injected into a wetgrinder, and water is mixed therein as a solvent, then dispersionprocessing is applied thereto, to thereby obtain a slurry.

When the obtained coarse powder and the fine powder are mixed, aprescribed CD can be guaranteed in the bonded magnet obtained in thesubsequent step if the mixing ratio of the coarse particles is 70pts.wt. or more (the mixing ratio of the fine particles is 30 pts.wt. orless). On the other hand, if the mixing ratio of the coarse particles isless than 80 pts.wt. (the mixing ratio of the fine particles is 20pts.wt. or more), a prescribed Hc can be obtained in the bonded magnetobtained in the subsequent step.

The obtained slurry is filtered or dehydrated to obtain a cake, and thecake is dried in the air to obtain a dried cake. Then, the dried cake iscrushed, to thereby obtain a mixed powder.

-   (6) The Step of Adding a Mechanical Pulverizing Force to the    Obtained Mixed Powder to Obtain a Pulverized Mixture, and Applying    Annealing to the Obtained Pulverized Mixture

A mechanical pulverizing force is added to the obtained mixed powder bycrushing the dried cake. Specifically, the mechanical pulverizing forceis the force caused by a pulverization processing in which a vibrationball mill with a capacity of 2 to 4 L, preferably 3 L, and power of 0.3to 0.5 kW, preferably 0.4 kW, is loaded with steel balls having adiameter of 8 to 14 min, preferably 12 mm as media, so thatpulverization is performed at a rotation number of 1700 to 1900 rpm,preferably 1800 rpm and an amplitude of 7 to 9 mm, preferably 8 mm for20 to 100 minutes as a processing time, or the force equivalent thereto.The vibration ball is preferably made of stainless steel. As apreferable specific example, Uras Vibrator KEC-8-YH by MURAKAMI SEMIMFG. CO., LTD. can be given.

A grinder is not particularly specified regarding the type and whetherit is a batch system or a continuous system, as long as it is thevibration ball mill by which a pulverization strength similar to abovecan be obtained.

For example by using a vibration mill as the vibration grinder, andapplying dry-type pulverization to the obtained mixed powder for 20minutes or more, the shapes of the particles of fine powder sintered atthe second temperature (lower temperature) can be unexpectedlyequalized. Then, the equalization of the particle shapes is consideredto contribute to improvement of the fluidity of the ferrite powder orimprovement of the fluidity of the compound. On the other hand, when thedry-type pulverization time is 100 minutes or less, crystal strain inthe particles (coarse particles and fine particles) of the mixed powdercaused by the pulverization is not excessive, thus making it easy toeliminate only the crystal strain in the subsequent step.

The dry-type pulverization may be the batch system or may be thecontinuous system. When the dry-type pulverization is performed by thecontinuous system, frequency, amplitude, supply amounts, andpulverization aids, etc., may be adjusted, to obtain the pulverizationstrength equivalent to that of the batch system.

After the mechanical pulverizing force is added to the mixed powder ofthe coarse powder and the fine powder, annealing is applied to the mixedpowder at a temperature of 940 to 990° C. for 5 to 60 minutes, tothereby obtain the ferrite powder of the present invention.

-   3.) Compound Filled with the Ferrite Powder of the Present Invention

In order to achieve the high coercive force in the ferrite bonded magnetwhich is an original object of the invention, it is found by theinventors of the present invention that MFR value is critical in thecompound which is the mixture of the ferrite powder and resin.

This is because the following problem is found by the inventors of thepresent invention: when F.C. of the ferrite powder in the compound isincreased, MFR of the compound is decreased, resulting in deteriorationof a molding performance when producing the magnet.

It is also found that high MFR value of 55 g/10 min or more can beobtained in the compound having high filling rate of F.C. 92.7 mass %for example, the compound being obtained by mixing and kneading theferrite powder and nylon resin.

The step of producing the compound will be described hereafter.

The ferrite powder, a coupling agent, a lubricant, and resin areweighed, injected and mixed into a mixer, etc., to thereby obtain amixture.

At this time, the ferrite powder is weighed so as to be a desired F.C.value. The coupling agent, for example, silane coupling agent ispreferably weighed by about 0.5 to 1.0 mass %. As the lubricant, forexample VPN-212P (manufactured by Henkel) can be preferably used, andabout 0.5 to 1.0 mass % is weighed. As the resin, for example nylon-6,etc., can be preferably used.

The obtained mixture is heated so that the resin is heated and melted,and the mixture is kneaded to obtain the compound. It is convenient toknead the mixture in a prescribed size and form it into a pellet.

-   4.) Bonded Magnet Obtained by Molding the Compound of the Present    Invention

By molding the compound of the present invention as described above, themolding product highly filled with the ferrite powder of the presentinvention can be easily obtained.

As a result, it is also found that the bonded magnet having a highmagnetic force with BH_(max) being the value of 2.4 to 3.0 MGOe can beeasily produced.

The step of producing the bonded magnet will be described hereafter.

The kneaded pellet of a prescribe size is injected into an injectionmolding machine, and a molding product of a desired size isinjection-molded at a prescribed temperature and under a moldingpressure while applying a magnetic field, to thereby obtain the bondedmagnet of the present invention.

The present invention is characterized in a manufacturing process asfollows: the sintering temperature during production of a fine particlepowder is set in a prescribed range, and the dry-type pulverization isstrengthened. By setting the sintering temperature in a prescribed rangeduring production of the fine particle powder, the coercive force of theobtained magnetic powder can be increased. Further, by strengthening thedry-type pulverization, the shapes of the fine particles can beequalized, and as a result, the fluidity and the orientation of thecompound can be high at the time of creating the bonded magnet, and as aresult, it was possible to guarantee a high MFR value of 55 g/10 min ormore in a bonded magnet with a high density filling value of 92.7 mass %for example.

EXAMPLES Example 1

-   1.) Production of the Ferrite Powder According to Example 1-   (1) The Step of Granulating a Plurality of Ferrite Raw Materials    Including Iron Oxide, and Obtaining a First Granulated Material

Iron oxide and strontium carbonate were weighed in a molar ratio of ironoxide 5.87:strontium carbonate 1. 0.17 mass % of boric acid and 2.36mass % of potassium chloride were added and mixed into the weighedmaterial, and thereafter water was added thereto and the mixture wasgranulated into a spherical shape with a diameter of 3 to 10 mm, tothereby obtain a first granulated material.

-   (2) The Step of Sintering the Obtained First Granulated Material at    a First Temperature, and Obtaining a Coarse Powder of the Sintered    Product

The granulated material was sintered at 1265° C. for 20 minutes underflowing atmosphere of air in a rotary kiln, to thereby obtain a sinteredproduct. A bulk density of the sintered product was 1.6 g/cm³, and itwas confirmed that there was almost no progress of sintering betweenparticles.

Treatment was applied to the sintered product by a roller mill, tothereby obtain a coarse powder of the sintered product.

-   (3) The Step of Granulating a Plurality of Ferrite Raw Materials    Including Iron Oxide, and Obtaining a Second Granulated Material

Iron oxide and strontium carbonate were weighed in a molar ratio of ironoxide 5.5:strontium carbonate 1. Water was added thereto and the mixturewas granulated into a spherical shape with a diameter of 3 to 10 mm, tothereby obtain a second granulated material.

-   (4) The Step of Sintering the Obtained Second Granulated Material at    a Second Temperature Lower than the First Temperature, and Obtaining    a Fine Powder of the Sintered Product

The granulated material was sintered at 970° C. for 20 minutes underflowing atmosphere of air in a rotary kiln, to thereby obtain a sinteredproduct. Treatment was applied to the sintered product by a roller mill,to thereby obtain a fine powder which is a sintered product.

-   (5) The Step of Mixing the Obtained Coarse Powder and the Fine    Powder

The obtained coarse powder (80 pts.wt.), the obtained fine powder (20pts.wt.), and tap water (150 pts.wt.) were weighed, and injected into acontainer having a stirring blade, and were stirred for 20 minutes andnixed, to thereby obtain a slurry in which particles of the coarsepowder and the fine powder were dispersed. Then, the slurry was filteredand dried (at 150° for 10 hours in the air), to thereby obtain a drycake.

-   (6) The Step of Adding a Mechanical Pulverizing Force to the Mixed    Powder of the Coarse Powder and the Fine Powder and Further Applying    Annealing Thereto

The obtained dry cake was pulverized by a vibration ball mill (UrasVibrator KEC-8-YH by MURAKAMI SEIKI MFG. CO., LTD.). The pulverizationprocessing was performed for 28 minutes under a pulverization processingcondition of a rotation number: 1800 rpm, an amplitude: 8 mm, and usinga steel ball with a diameter of 12 mm as media. The pulverized mixedpowder was annealed for 30 minutes at 950° C. in the air, to therebyobtain the ferrite powder of example 1.

The abovementioned production condition is described in table 1.

FIG. 1A shows a particle size distribution curve of the obtained ferritepowder of example 1, and FIG. 2A shows a cumulative particle sizedistribution curve thereof, respectively shown by solid lines. Wherein,it is confirmed that the particle size distribution curve has a twomountain-like peaks.

In the present invention, the particle size distribution curve indicatesa frequency distribution curve obtained using a particle sizedistribution measurement device.

Further, in the examples and comparative examples of the presentinvention, a maximum value of the peak is not necessarily required to bea completely independent peak. Namely, when a shoulder is present in askirt portion of the peak, the shoulder is considered to be anotherpeak.

Table 2 shows the average particle size in the particles of the ferritepowder, peak particle sizes at two mountains (peak particle size (1) andpeak particle size (2)), cumulative distribution value at particle size0.62 μm in the cumulative particle size distribution curve, cumulativedistribution value at particle size 0.74 μm in the cumulative particlesize distribution curve, and values of SSA, CD, and p-iHc. Table 2 showsa residual magnetic flux density (p-Br) of a compressed molding,saturation magnetization (σs) of the ferrite powder, and coercive force(Hc) of the ferrite powder.

Then, from the values of table 2, it is confirmed that the ferritepowder of example 1 guarantees the average particle size and the valuesof SSA, CD, and p-iHc.

-   2.) Production of the Green Compound-   (1) Production of the Pellet with F.C. of 92.7 Mass %

The ferrite powder 92.7 pts.wt. of example 1, silane coupling agent 0.8pts.wt., lubricant 0.8 pts.wt., and nylon-6 (powdery shape) 5.7 pts.wt.were weighed, injected and mixed into a mixer, to thereby obtain amixture. The obtained mixture was kneaded at 230° C., to thereby obtaina kneaded pellet (1) of example 1 with an average diameter of 2 mm as apulverized product of the compound. A mass ratio (ferritepowder/nylon-6) in the pellet was 16.3.

In this case, table 3 shows the values of MFR of the kneaded pellet (1)of example 1.

-   3. ) Molding of the Compound and Production of the Bonded Magnet-   (1) Production of the Bonded Magnet with F.C. of 92.7 Mass % and 4.3    kOe Orientation.

The obtained kneaded pellet (1) was injected into an injection-moldingmachine (by SUMITOMO Heavy Industries. Ltd,), and was injection-moldedat a temperature of 290° C., with a molding pressure of 8.5 N/mm² in amagnetic field of 4.3 kOe, to thereby obtain the bonded magnet ofexample 1 (with F.C. of 92.7 mass % and 4.3 kOe orientation) with acolumnar shape of diameter 15 mm×height 8 mm (an orientation directionof the magnetic field is a direction along a central axis of a column).

In this case, table 3 shows the values of Br, iHc (inj-iHc), and of thebonded magnet (1) of example 1 (with F.C. of 92.7 mass % and 4.3 kOeorientation).

-   (2) Production of the Bonded Magnet with F.C. of 92.7 Mass % and 12    kOe Orientation.

Bonded magnet (2) of example 1 (with F.C. of 92.7 mass %. and 12 kOeorientation) was obtained by performing a similar operation other thanthe point that the kneaded pellet (1) of example 1 was used and themagnetic field during injection molding was set to 12 kOe.

In this case, table 3 shows the values of Br, iHc, and BH_(max) of thebonded magnet (2) of example 1 (with F.C. of 92.7 mass %. and 12 kOeorientation).

-   (3) Production of the Pellet with F.C. of 93.3 Mass %

The ferrite powder 93.3 pts.wt. of example 1, silane coupling agent 0.7pts.wt., lubricant 0.8 pts.wt., and nylon-6 (powdery shape) 5.2 pts.wt.were weighed, injected and mixed into a mixer, to thereby obtain amixture. The obtained mixture was kneaded at 230° C., to thereby obtaina kneaded pellet (2) of example 1 with an average diameter of 2 mm. Amass ratio (ferrite powder nylon-6) in the pellet was 17.9.

In this case, table 3 shows the values of MFR of the kneaded pellet (2)of example 1.

-   3.) Molding of the Compound and Production of the Bonded Magnet-   (1) Production of the Bonded Magnet with F.C. of 93.3 Mass % and 4.3    kOe Orientation

The bonded magnet (3) of example 1 (with F.C. of 93.3 mass %. and 4 kOeorientation) was obtained by performing the similar operation other thanthe point that kneaded pellet (2) of example 1 was used and the magneticfield during injection molding was set to 4.3 kOe.

In this case, table 3 shows the values of Br, iHc, and BH_(max) of thebonded magnet (3) of example 1 (with F.C. of 93.3 mass % and 4.3 kOeorientation).

-   4.) Measurement Method

Explanation will be given for a measurement method of each kind ofproperty of the ferrite powder, pellet, and bonded magnet produced inexample 1. The same thing can be said for examples 2 to 6 andcomparative examples 1 to 3 described hereafter.

-   <Particle Size Distribution>

Regarding the particle size distribution of the ferrite powder, theparticle size distribution on a volume basis can be measured underconditions of focallength=20 mm, a dispersion pressure: 5.0 bar, and asuction pressure: 130 mbar, using a dry laser diffraction particle sizedistribution measuring device (HELOS & RODLS by Japan Laser Corporation)

Further, three measurement points near the maximum value in the particlesize distribution curve were approximated by a quadratic function, and aparticle size which was the maximum value of the quadratic function wasset as the peak particle size.

-   <Specific Surface Area (SSA)>

SSA of the ferrite powder was measured using a Monosorb manufactured byYuasa Ionics Corporation, based on BET method.

-   <Compression Density (CD)>

Regarding CD of the ferrite powder, a cylindrical mold having an innerdiameter of 2.54 cmϕ was filled with the ferrite powder 10 g, which wasthen compressed with a pressure of 1 ton/cm². The density of the ferritepowder at this time was used as CD for measuring the particle sizedistribution.

-   <Coercive Force (p-iHc) of the Compressed Molding>

p-iHc of the ferrite powder was measured by the following procedure.

(1) The ferrite powder 8 g and polyester resin (P-resin by JapanGeoscience Corp.) 0.4 cc were kneaded in a mortar.

(2) The kneaded product 7 g was injected into a mold having an innerdiameter of 15 mmϕ, which was then compressed for 40 seconds at apressure of 2 tons/cm².

(3) A molding product was extracted from the mold and dried for 30minutes at 150° C., and thereafter p-iHc of the ferrite powder wasmeasured in a measurement magnetic field of 10 kOe by BH tracer (TRF-5BHmanufactured by Toei Kogyo).

-   <Magnetic Property>

Magnetic properties of the ferrite powder were measured in themeasurement magnetic field of 10 kOe, using VSM (VSMP-7-15 manufacturedby Toei Industry Co., Ltd.), by injecting the ferrite powder 20 mg andparaffin 30 mg into a cell attached to an apparatus which was thenheated to 80° C. to melt the paraffin, and cooled to a room temperature,to thereby randomly solidify sample particles, and calculate σs (emu/g)and iHc (Oe). It should be noted that 1 Oe corresponds to ¼π×10³[A/m],

-   <Fluidity (MFR)>

MFR of the kneaded pellet was provided for a melt flow indexer ((meltflow indexer C-5059D2 (in conformity to JISK-7210) by Toyo SeikiSeisakusho), and MFR (unit g/10 min) was obtained by measuring a weightextruded with a load of 10 kg at 270° C. and converting it to anextrusion amount per 10 minutes.

In this specification, MFR is the value measured by the followingprocedures (1) to (3).

(1) The magnetic powder 92.7 pts.wt. or 93.3 pts.wt. to be measured,silane coupling agent 0.7 pts.wt., lubricant 0.8 pts.wt., and nylong-6(powdery shape) 5.7 pts.wt., were stirred by a mixer.

(2) The obtained mixture was kneaded at 230° C., and formed into apellet with an average diameter of 2 mm, as a crushed compound.

(3) The pellet obtained by above (2) was provided for the melt flowindexer, and the weight extruded in 10 minutes under the load of 10 kgat 270° C. was measured as MFR (unit g/10 min).

-   <Magnetic Properties of the Molding Product>

Magnetic properties of the molding product were evaluated by thefollowing procedures.

(1) The kneaded pellet was injection-molded under molding pressure of8.5 N/mm² at a temperature of 290° C. in the magnetic field of 4.3 kOe,using an injection molding machine (manufactured by Sumitomo HeavyIndustries), to thereby obtain a columnar molding product with diameter15 mm×height 8 mm (the orientation direction of the magnetic field isthe direction along the central axis of the column).

(2) The magnetic properties of the columnar molding product weremeasured in a measurement magnetic field of 10 kOe by BH tracer (TRF-5BHmanufactured by Toei Kogyo).

Example 2

-   1.) Production of the Mixed Powder (Ferrite Powder)

The ferrite powder of example 2 was obtained by performing the similaroperation as example 1, as described in “(5) the step of mixing theobtained coarse powder and the fine powder” was set as coarse powder: 75pts.wt. and the fine powder: 25 pts.wt, other than the point that theratio of the coarse powder and the fine powder was set as coarse powder:75 pts.wt. and the fine powder: 25 pts.wt.

The abovementioned production conditions are described in table 1.

FIG. 1A shows the particle size distribution curve of the obtainedferrite powder of example 2, and FIG. 2A shows the cumulative particlesize distribution curve thereof, respectively by short broken lines,wherein it was confirmed that the particle size distribution curves hadtwo mountain-like peaks. Table 2 shows the average particle size of theferrite powder, the peak particle size at two mountains, the cumulativedistribution value at a particle size of 0.62 μm in the cumulativeparticle size distribution curve, and the cumulative distribution valueat a particle size of 0.74 μm in the cumulative particle sizedistribution curve, and the values of SSA, CD, σs, Hc, p-iHc, and p-Br.

-   2.) Production of the Compound

(1) Production of the Pellet with F.C. of 92.7 Mass %.

Kneaded pellet (1) of example 2 was obtained by performing the similaroperation as the kneaded pellet (1) of example 1, other than the pointthat the ferrite powder of example 2 was used.

In this case, table 3 shows the values of MFR of the kneaded pellet (1)of example 2.

-   (2) Production of the Pellet with F.C. of 93.3 Mass %.

Kneaded pellet (2) of example 2 was obtained by performing the similaroperation as the kneaded pellet (2) of example 1, other than the pointthat the ferrite powder of example 2 was used.

In this case, table 3 shows the values of MFR of the kneaded pellet (2)of example 2.

-   3.) Molding of the Compound and Production of the Bonded Magnet-   (1) Production of the Bonded Magnet with F.C. of 92.7 Mass %. and    4.3 kOe Orientation

Bonded magnet (1) of example 2 (with F.C. of 92.7 mass %. and 4.3 kOeorientation) was obtained by performing the similar operation as example1, other than the point that the kneaded pellet (1) of example 2 wasused.

In this case, table 3 shows the values of Br, iHc, and BH_(max) of thebonded magnet (1) of example 2 (with F.C. of 92.7 mass %. and 4.3 kOeorientation).

-   (2) Production of the Bonded Magnet with F.C. of 92.7 Mass %. and 12    kOe Orientation

Bonded magnet (2) of example 2 (with F.C. of 92.7 mass %. and 1.2 kOeorientation) was obtained by performing the similar operation as example1, other than the point that the kneaded pellet (2) of example 2 wasused.

In this case, table 3 shows the values of Br, iHc, and BH_(max) of thebonded magnet (2) of example 2 (with F.C. of 92.7 mass %. and 12 kOeorientation).

-   (3) Production of the Bonded Magnet with F.C. of 93.3 Mass %. and    4.3 kOe Orientation

Bonded magnet (3) of example 2 (with F.C. of 93.3 mass %. and 4.3 kOeorientation) was obtained by performing the similar operation as example1, other than the point that the kneaded pellet (2) of example 2 wasused.

In this case, table 3 shows the values of Br, iHc, and BH_(max) of thebonded magnet (3) of example 2 (with F.C. of 93.3 mass %. and 4.3 kOeorientation).

Example 3

-   1.) Production of the Mixed Powder (Ferrite Powder)

The ferrite powder of example 3 was obtained by performing a similaroperation as example 1, other than the point that other than the pointthat the first sintering temperature for sintering the coarse powder asdescribed in the step of “(2) sintering the obtained first granulatedmaterial at a first temperature, and obtaining a coarse powder of asintered product” was set to 1280° C., and the mixing ratio of thecoarse powder and the fine powder as described in the step of “(5)mixing the obtained coarse powder and fine powder” was set as the coarsepowder (75 pts.wt.), and the fine powder (25 pts.wt.).

The abovementioned production conditions are described in table 1.

FIG. 1A shows the particle size distribution curve of the obtainedferrite powder of example 3, and FIG. 2A shows the cumulative particlesize distribution curve thereof, respectively by long broken lines,wherein it was confirmed that the particle size distribution curves hadtwo mountain-like peaks. Table 2 shows the average particle size of theferrite powder, the peak particle sizes at two mountains, the cumulativedistribution value at a particle size of 0.62 μm in the cumulativeparticle size distribution curve, and the cumulative distribution valueat a particle size of 0.74 μm in the cumulative particle sizedistribution curve, and the values of SSA, CD, σs, Hc, p-iHc, and p-Br.

-   2.) Production of the Compound-   (1) Production of the Pellet with F.C. of 92.7 Mass %.

Kneaded pellet (1) of example 3 was obtained by performing the similaroperation as the kneaded pellet (1) of example 1, other than the pointthat the ferrite powder of example 3 was used.

In this case, table 3 shows the values of MFR of the kneaded pellet (1)of example 3.

-   1.) Production of the Compound-   (1) Production of the Pellet with F.C. of 93.3 Mass %.

Kneaded pellet (2) of example 3 was obtained by performing the similaroperation as the kneaded pellet (2) of example 1, other than the pointthat the ferrite powder of example 3 was used.

In this case, table 3 shows the values of MFR of the kneaded pellet (2)of example 3.

-   3.) Molding of the Compound and Production of the Bonded Magnet-   (1) Production of the Bonded Magnet with F.C. of 92.7 Mass %. and    4.3 kOe Orientation

The bonded magnet (1) of example 3 (with F.C. of 92.7 mass %. and 4.3kOe orientation) was obtained by performing the similar operation asexample 1, other than the point that the kneaded pellet (1) of example 3was used.

In this case, table 3 shows the values of Br, iHc, and BH_(max) of thebonded magnet (1) of example 3 (with F.C. of 92.7 mass %. and 4.3 kOeorientation).

-   (2) Production of the Bonded Magnet with F.C. of 92.7 Mass %. and 12    kOe Orientation

The bonded magnet (2) of example 3 (F.C. of 92.7 mass %. and 12 kOeorientation) was obtained by performing the similar operation as example1, other than the point that the kneaded pellet (1) of example 3 wasused.

In this case, table 3 shows the values of Br, iHc, and BH_(max) of thebonded magnet (2) of example 3 (with. F.C. of 92.7 mass %. and 12 kOeorientation).

-   (3) Production of the Bonded Magnet with F.C. of 93.3 Mass %. and    4.3 kOe Orientation

Bonded magnet (3) of example 3 (with F.C. of 93.3 mass %. and 4.3 kOeorientation) was obtained by performing the similar operation as example1, other than the point that the kneaded pellet (2) of example 3 wasused.

In this case, table 3 shows the values of Br, iHc, and BH_(max) of thebonded magnet (3) of example 3 (with F.C. of 93.3 mass %. and 4.3 kOeorientation).

Example 4

-   1.) Production of the Mixed Powder (Ferrite Powder)

The ferrite powder of example 4 was obtained by performing the similaroperation as example 1, other than the point that the sinteringtemperature for sintering the coarse powder as described in “(2) Thestep of sintering the obtained first granulated material at a firsttemperature, and obtaining a coarse powder of the sintered product” wasset to 1250° C. and the mixing ratio of the coarse powder and the finepowder was set as the coarse powder (75 pts.wt.), and the fine powder(25 pts.wt.), and an annealing temperature as described in the step of“(6) adding a mechanical pulverizing force to the obtained mixed powderto obtain a pulverized mixture, and applying annealing to the obtainedpulverized mixture” was set to 970° C.

The abovementioned production conditions are described in table 1.

FIG. 1B shows the particle size distribution curve of the obtainedferrite powder of example 4, and FIG. 2B shows the cumulative particlesize distribution curve thereof, respectively by short broken lines,wherein it was confirmed that the particle size distribution curves hadtwo mountain-like peaks. Table 2 shows the average particle size in theparticles of the ferrite powder, the peak particle sizes at twomountains, the cumulative distribution value at particle size 0.62 μm inthe cumulative particle size distribution curve, the cumulativedistribution value at particle size 0.74 μm in the cumulative particlesize distribution curve, and the values of SSA, CD, σs, Hc, p-iHc, andp-Br.

-   2.) Production of the Compound-   (1) Production of the Pellet with F.C. of 92.7 Mass %.

Kneaded pellet (1) of example 4 was obtained by performing the similaroperation as the kneaded pellet (1) of example 1, other than the pointthat the ferrite powder of example 4 was used.

In this ease, table 3 shows the values of MFR of the kneaded pellet (1)of example 4.

-   (2) Production of the Pellet with F.C. of 93.3 Mass %.

Kneaded pellet (2) of comparative example 4 was obtained by performingthe similar operation as the kneaded pellet (2) of example 1, other thanthe point that the ferrite powder of example 4 was used.

In this case, table 3 shows the values of MFR of the kneaded pellet (2)of example 4.

-   3.) Molding of the Compound and Production of the Bonded Magnet-   (1) Production of the Bonded Magnet with F.C. of 92.7 Mass % and 4.3    kOe Orientation

The bonded magnet (1) of example 4 (with F.C. of 92.7 mass %. and 4.3kOe orientation) was obtained by performing the similar operation asexample 1, other than the point that kneaded pellet (1) of example 4 wasused.

In this case, table 3 shows the values of Br, iHc, and BH_(max) of thebonded magnet (1) of example 4 (with F.C. of 92.7 mass % and 4.3 kOeorientation).

-   (2) Production of the Bonded Magnet with F.C. of 92.7 Mass %. and 12    kOe Orientation

The bonded magnet (2) of example 4 (F.C. of 92.7 mass %. and 12 kOeorientation) was obtained by performing the similar operation as example1, other than the point that the kneaded pellet (1) of example 4 wasused.

In this case, table 3 shows the values of Br, iHc, and BH_(max) of thebonded magnet (2) of example 4 (with F.C. of 92.7 mass %. and 12 kOeorientation).

-   (3) Production of the Bonded Magnet with F.C. of 93.3 Mass %. and    4.3 kOe Orientation

Bonded magnet (3) of example 4 (with of 93.3 mass %. and 4.3 kOeorientation) was obtained by performing the similar operation as example1, other than the point that the kneaded pellet (2) of example 4 wasused.

In this case, table3 shows the values of Br, iHc, and BH_(max) of thebonded magnet (3) of example 4 (with F.C. of 93.3 mass %. and 4.3 kOeorientation).

Example 5

-   1.) Production of the Mixed Powder (Ferrite Powder)

The ferrite powder of example 5 was obtained by performing the similaroperation as example 1, other than the point that the sinteringtemperature for sintering the coarse powder as described in “(2) Thestep of sintering the obtained first granulated material at a firsttemperature, and obtaining a coarse powder of the sintered product” wasset to 1250° C. and the mixing ratio of the coarse powder and the finepowder was set as the coarse powder (75 pts.wt.), and the fine powder(25 pts.wt.), and an annealing temperature as described in the step of“(6) adding a mechanical pulverizing force to the obtained mixed powderto obtain a pulverized mixture, and applying annealing to the obtainedpulverized mixture” was set to 955° C.

The abovementioned production conditions are described in table 1.

FIG. 1B shows the particle size distribution curve of the obtainedferrite powder of example 5, and FIG. 2B shows the cumulative particlesize distribution curve thereof, respectively by broken lines, whereinit was confirmed that the particle size distribution curves had twomountain-like peaks. Table 2 shows the average particle size in theparticles of the ferrite powder, the peak particle sizes at twomountains, the cumulative distribution value at particle size 0.62 μm inthe cumulative particle size distribution curve, the cumulativedistribution value at particle size 0.74 μm in the cumulative particlesize distribution curve, and the values of SSA, CD, σs, Hc, p-iHc, andp-Br.

-   2.) Production of the Compound-   (1) Production of the Pellet with F.C. of 92.7 Mass %.

Kneaded pellet (1) of example 5 was obtained by performing the similaroperation as the kneaded pellet (1) of example 1, other than the pointthat the ferrite powder of example 5 was used.

In this case, table 3 shows the values of MFR of the kneaded pellet (1)of example 5.

-   (2) Production of the Pellet with F.C. of 93.3 Mass %.

Kneaded pellet (2) of comparative example 5 was obtained by performingthe similar operation as the kneaded pellet (2) of example 1, other thanthe point that the ferrite powder of comparative example 5 was used.

In this case, table 3 shows the values of MFR of the kneaded pellet (2)of example 5.

-   (1) Production of the Bonded Magnet with F.C. of 92.7 Mass %. and    4.3 kOe Orientation

The bonded magnet (1) of example 5 (with F.C. of 92.7 mass %. and 4.3kOe orientation) was obtained by performing the similar operation otherthan the point that kneaded pellet (1) of example 5 was used.

In this case, table 3 shows the values of Br, and BH_(max) of the bondedmagnet (1) of example 5 (with. F.C. of 92.7 mass % and 4.3 kQeorientation).

-   (2) Production of the Bonded Magnet with F.C. of 92.7 Mass %. and 12    kOe Orientation

The bonded magnet (2) of example 5 (F.C. of 92.7 mass %. and 12 kOeorientation) was obtained by performing the similar operation as example1, other than the point that the kneaded pellet (1) of example 5 wasused.

In this case, table 3 shows the values of Br, iHc , and BH_(max) of thebonded magnet (2) of example 5 (with F.C. of 92.7 mass %. and 12 kOeorientation).

-   (3) Production of the Bonded Magnet with F.C. of 93.3 Mass %. and    4.3 kOe Orientation

Bonded magnet (3) of example 5 (with F.C. of 93.3 mass %. and 4.3 kOeorientation) was obtained by performing the similar operation as example1, other than the point that the kneaded pellet (2) of example 5 wasused.

In this case, table 3 shows the values of Br, iHc, and BH_(max) of thebonded magnet (3) of example 5 (with F.C. of 93.3 mass %. and 4.3 kOeorientation).

Example 6

-   1.) Production of the Mixed Powder (Ferrite Powder)

The ferrite powder of example 6 was obtained by performing the similaroperation as example 1, other than the point that the sinteringtemperature for sintering the coarse powder as described in “(2) Thestep of sintering the obtained first granulated material at a firsttemperature, and obtaining a coarse powder of the sintered product” wasset to 1250° C. and the mixing ratio of the coarse powder and the finepowder was set as the coarse powder (75 pts.wt.), and the fine powder(25 pts.wt.), and an annealing temperature as described in the step of“(6) adding a mechanical pulverizing force to the obtained mixed powderto obtain a pulverized mixture, and applying annealing to the obtainedpulverized mixture” was set to 940° C.

The abovementioned production conditions are described in table 1.

FIG. 1B shows the particle size distribution curve of the obtainedferrite powder of example 6, and FIG. 28 shows the cumulative particlesize distribution curve thereof, respectively by long broken lines,wherein it was confirmed that the particle size distribution curves hadtwo mountain-like peaks. Table 2 shows the average particle size in theparticles of the ferrite powder, peak particle sizes at two mountains,the cumulative distribution value at particle size 0.62 μm in thecumulative particle size distribution curve, the cumulative distributionvalue at particle size 0.74 μm in the cumulative particle sizedistribution curve, and the values of SSA, CD, σs, Hc, p-iHc, and p-Br.

-   2.) Production of the Compound-   (1) Production of the Pellet with F.C. of 92.7 Mass %.

Kneaded pellet (1) of example 6 was obtained by performing the similaroperation as the kneaded pellet (1) of example 1, other than the pointthat the ferrite powder of comparative example 6 was used.

In this case, table 3 shows the values of MFR of the kneaded pellet (1)of example 6.

-   (2) Production of the Pellet with F.C. of 93.3 Mass %.

Kneaded pellet (2) of comparative example 6 was obtained by performingthe similar operation as the kneaded pellet (2) of example 1, other thanthe point that the ferrite powder of comparative example 6 was used.

In this case, table 3 shows the values of MFR of the kneaded pellet (2)of comparative example 6.

-   3.) Molding of the Compound and Production of the Bonded Magnet-   (1) Production of the Bonded Magnet with F.C. of 92.7 Mass %. and    4.3 kOe Orientation

Bonded magnet (1) of example 6 (with F.C. of 92.7 mass %. and 4.3 kOeorientation) was obtained by performing the similar operation as example1, other than the point that the kneaded pellet (1) of example 6 wasused.

In this case, table 3 shows the values of Br, iHc, and BH_(max) of thebonded magnet (1) of example 6 (with F.C. of 92.7 mass %. and 4.3 kOeorientation).

-   (2) Production of the Bonded Magnet with F.C. of 92.7 Mass %. and 12    kOe Orientation

The bonded magnet (2) of example 6 (F.C. of 92.7 mass %. and 12 kOeorientation) was obtained by performing the similar operation as example1, other than the point that the kneaded pellet (1) of example 6 wasused.

In this case, table 3 shows the values of Br, iHc, and BH_(max) of thebonded magnet (2) of example 6 (with of 92.7 mass %. and 12 kOeorientation).

-   (3) Production of the Bonded Magnet with F.C. of 93.3 Mass %. and    4.3 kOe Orientation

Bonded magnet (3) of example 6 (with F.C. of 93.3 mass %. and 4.3 kOeorientation) was obtained by performing the similar operation as example1, other than the point that the kneaded pellet (2) of example 6 wasused.

In this case, table 3 shows the values of Br, iHc, and BH_(max) of thebonded magnet (3) of example 6 (with F.C. of 93.3 mass %. and 4.3 kOeorientation).

Comparative Example 1

-   1.) Production of the Mixed Powder (Ferrite Powder)

The ferrite powder of comparative example 1 was obtained by performingthe similar operation as example 1, other than the point that the firstsintering temperature as described in the step of “(2) sintering theobtained first granulated material at a first temperature, and obtaininga coarse powder of a sintered product” was set to 1235° C., and thesintering temperature under flowing atmosphere of air in a rotary kilnas described in the step of “(4) sintering the obtained secondgranulated material at a second temperature lower than the firsttemperature, and obtaining fine powder of the sintered product” was setto 1070° C., and the mixing ratio of the coarse powder and the finepowder was set as the coarse powder (70 pts.wt.), and the fine powder(30 pts.wt.), and the pulverization processing time by the vibrationball mill for the obtained mixed powder as described in the step of “(6)adding a mechanical pulverizing force to the obtained mixed powder toobtain a pulverized mixture, and applying annealing to the obtainedpulverized mixture” was set to 14 minutes, and an annealing temperatureas described in this step was set to 910° C.

The abovementioned production conditions are described in table 1.

FIG. 1C shows the particle size distribution curve of the obtainedferrite powder of comparative example 1, and FIG. 2C shows thecumulative particle size curve thereof, by one-dot chain linerespectively, and it was confirmed that the particle size distributioncurve had two mountain-like peaks. Table 2 shows the average particlesize in the particles of the ferrite powder, the peak particle sizes attwo mountains, the cumulative distribution value at particle size 0.62μm in the cumulative particle size distribution curve, the cumulativedistribution value at particle size 0.74 μm in the cumulative particlesize distribution curve, and the values of SSA, CD, σs, Hc, p-iHc, andp-Br.

-   2.) Production of the Compound (1) Production of the Pellet with    F.C. of 92.7 Mass %.

Kneaded pellet (1) of comparative example 1 was obtained by performingthe similar operation as the kneaded pellet (1) of example 1, other thanthe point that the ferrite powder of comparative example 1 was used.

In this case, table 3 shows the values of MFR of the kneaded pellet (1)of comparative example 1.

-   (2) Production of the Pellet with F.C. of 93.3 Mass %.

Kneaded pellet (2) of comparative example 1 was attempted to be obtainedby performing the similar operation as the kneaded pellet (2) of example1, other than the point that the ferrite powder of comparative example 1was used. However, it was impossible to perform kneading.

-   3.) Molding of the Compound and Production of the Bonded Magnet-   (1) Production of the Bonded Magnet with F.C. of 92.7 Mass %. and    4.3 kOe Orientation

Bonded magnet (1) of comparative example 1(with F.C. of 92.7 mass %. and4.3 kOe orientation) was obtained by performing the similar operation asexample 1, other than the point that the kneaded pellet (1) ofcomparative example 1 was used.

In this case, table 3 shows the values of Br, iHc, and BH_(max) of thebonded magnet (1) of comparative example 1 (with F.C. of 92.7 mass %.and 4.3 kOe orientation).

Comparative Example 2

-   1.) Production of the Mixed Powder (Ferrite Powder)

The ferrite powder of comparative example 2 was obtained by performingthe similar operation as example 1, other than the point that the firstsintering temperature as described in the step of “(2) sintering theobtained first granulated material at a first temperature, and obtaininga coarse powder of a sintered product” was set to 1235° C., and thesintering temperature under flowing atmosphere of air in a rotary kilnas described in the step of “(4) sintering the obtained secondgranulated material at a second temperature lower than the firsttemperature, and obtaining fine powder of the sintered product” was setto 1070° C., and the mixing ratio of the coarse powder and the finepowder as described in the step of “(5) mixing the obtained coarsepowder and fine powder” was set as the coarse powder (70 pts.wt.), andthe fine powder (30 pts.wt.), and the pulverization processing time bythe vibration ball mill for the obtained mixed powder as described inthe step of “(6) adding a mechanical pulverizing force to the obtainedmixed powder to obtain a pulverized mixture was set to 14 minutes, andan annealing temperature as describe in this step was set to 940° C.

The abovementioned production conditions are described in table 1.

FIG. 1C shows the particle size distribution curve of the obtainedferrite powder of comparative example 2, and FIG. 2C shows thecumulative particle size curve thereof, by two-dot chain linerespectively, and it was confirmed that the particle size distributioncurve had two mountain-like peaks. Table 2 shows the average particlesize in the particles of the ferrite powder, the peak particle sizes attwo mountains, the cumulative distribution value at particle size 0.62μm the cumulative particle size distribution curve, the cumulativedistribution value at particle size 0.74 μm in the cumulative particlesize distribution curve, and the values of SSA, CD, σs, Hc, p-iHc, andp-Br.

-   2.) Production of the Compound-   (1) Production of the Pellet with F.C. of 92.7 Mass %.

Kneaded pellet (1) of comparative example 2 was obtained by performingthe similar operation as the kneaded pellet (1) of example 1, other thanthe point that the ferrite powder of comparative example 2 was used.

In this case, table 3 shows the values of MFR of the kneaded pellet (1)of comparative example 2.

-   (2) Production of the Pellet with F.C. of 93.3 Mass %.

Kneaded pellet (2) of comparative example 2 was attempted to be obtainedby performing the similar operation as the kneaded pellet (2) of example1, other than the point that the ferrite powder of comparative example 2was used. However, it was impossible to perform kneading.

-   3.) Molding of the Compound and Production of the Bonded Magnet-   (1) Production of the Bonded Magnet with F.C. of 92.7 Mass %. and    4.3 kOe Orientation

Bonded magnet (1) of comparative example 2 (with F.C. of 92.7 mass %,and 4.3 kOe orientation) was obtained by performing the similaroperation as example 1, other than the point that the kneaded pellet (1)of comparative example 2 was used.

In this case, table 3 shows the values of Br, iHc, and BH_(max) of thebonded magnet (1) of comparative example 2 (with F.C. of 92.7 mass %.and 4.3 kOe orientation).

-   1.) Production of the Mixed Powder (Ferrite Powder)

The ferrite powder of comparative example 3 was obtained by performingthe similar operation as example 1, other than the point that the firstsintering temperature for sintering the coarse powder as described inthe step of “(2) sintering the obtained first granulated material at afirst temperature, and obtaining a coarse powder of a sintered product”was set to 1230° C., and the sintering temperature under flowingatmosphere of air in a rotary kiln as described in the step of “(4)sintering the obtained second granulated material at a secondtemperature lower than the first temperature, and obtaining fine powderof the sintered product” was set to 1070° C., and the mixing ratio ofthe coarse powder and the fine powder as described in the step of “(5)mixing the obtained coarse powder and fine powder” was set as the coarsepowder (70 pts.wt.), and the fine powder (30 pts.wt.), and thepulverization processing time by the vibration ball mill for theobtained mixed powder as described in the step of “(6) adding amechanical pulverizing force to the obtained mixed powder to obtain apulverized mixture, and applying annealing o the obtained pulverizedmixture” was set to 14 minutes, and an annealing temperature asdescribed in this step was set to 965° C.

The abovementioned production conditions are described in table 1.

FIG. IC shows the particle size distribution curve of the obtainedferrite powder of comparative example 3, and FIG. 2C shows thecumulative particle size curve thereof, by short two-dot chain linerespectively, and it was confirmed that the particle size distributioncurve had two mountain-like peaks. Table 2 shows the average particlesize in the particles of the ferrite powder, peak particle sizes at twomountains, the cumulative distribution value at particle size 0.62 μm inthe cumulative particle size distribution curve, the cumulativedistribution value at particle size 0.74 μm in the cumulative particlesize distribution curve, and the values of SSA, CD, σs, Hc, p-iHc, andp-Br.

-   2.) Production of the Compound-   (1) Production of the Pellet with F.C. of 92.7 Mass %.

Kneaded pellet (1) of comparative example 3 was obtained by performingthe similar operation as the kneaded pellet (1) of example 1, other thanthe point that the ferrite powder of comparative example 3 was used.

In this case, table 3 shows the values of MFR of the kneaded pellet (1)of comparative example 3.

-   (2) Production of the Pellet with F.C. of 93.3 Mass %.

Kneaded pellet (2) of comparative example 3 was attempted to be obtainedby performing the similar operation as the kneaded pellet (2) of example1, other than the point that the ferrite powder of comparative example 3was used. However, it was impossible to perform kneading,

-   3.) Molding of the Compound and Production of the Bonded Magnet-   (1) Production of the Bonded Magnet with F.C. of 92.7 Mass %. and    4.3 kOe Orientation

Bonded magnet (1) of comparative example 3 (with F.C. of 92.7 mass %.and 4.3 kOe orientation) was obtained by performing the similaroperation as example 1, other than the point that the kneaded pellet (1)of comparative example 3 was used.

In this case, table 3 shows the values of Br, iHc, and BH_(max) of thebonded magnet (1) of comparative example 3 (with F.C. of 92.7 mass %.and 4.3 kOe orientation).

-   (2) Production of the Bonded Magnet with F.C. of 92.7 Mass %. and 12    kOe Orientation

Bonded magnet (2) of comparative example 3 (with F.C. of 92.7 mass %.and 12 kOe orientation) was obtained by performing the similar operationas example 1, other than the point that the kneaded pellet (2) ofcomparative example 3 was used.

In this case, table 3 shows the values of Br, iHc, and BH_(max) of thebonded magnet (2) of comparative example 3 (with F.C. of 92.7 mass %.and 12 kOe orientation).

Conclusion

The ferrite powder for bonded magnets of examples 1 to 6 realizes smallparticle size of the coarse powder and the fine powder by controllingthe sintering temperature for the ferrite powder, and obtains a highdispersibility by adding the mechanical pulverizing force to the mixedpowder of the coarse powder and the fine powder for a long time, andtherefore has SSA of 1.93 to 2.41 m²/g, CD of 3.58 to 3.66 g/cm³, andp-iHc of 2340 to 2640 Oe.

As a result, when the mixture of the ferrite powder for bonded magnetsof examples 1 to 6 and resin is kneaded, the ferrite bonded magnet canbe easily produced, with MFR being 68.9 to 101.7 g/10 min, and BH_(max)being 2.57 to 2.68 MGOe in F.C. of 92.7 mass %. Also, the ferrite bondedmagnet can be easily produced, with MFR being 30.2 to 50.7 g/10 min, andBH_(max) being 2.78 to 2.83 MGOe i F.C. of 93.3 mass %.

TABLE 1 Mixed powder Coarse powder Fine powder Coarse Dry-typeComposition Sintering Composition Sintering powder/ culveri- AnnealingFe₂O₃/ KCl Temper- Fe₂O₃/ Temper- Fine powder zation Temper- SrCO₃ Boricacid (Mass ature Time SrCO₃ ature Time Mixing ratio Time ature (Molarratio) (Mass %) %) (° C.) (Minute) (Molar ratio) (° C.) (Minute) (Pts.wt.) (Minute) (° C.) Example 1 5.87 0.17 2.36 1265 20 5.5 970 20 80/2028 950 Example 2 5.87 0.17 2.36 1265 20 5.5 970 20 75/25 28 950 Example3 5.87 0.17 2.36 1280 20 5.5 970 20 75/25 28 950 Example 4 5.87 0.172.36 1250 20 5.5 970 20 75/25 28 970 Example 5 5.87 0.17 2.36 1250 205.5 970 20 75/25 28 955 Example 6 5.87 0.17 2.36 1250 20 5.5 970 2075/25 28 940 Comparative 5.87 0.17 2.36 1235 20 5.5 1070 20 70/30 14 910example 1 Comparative 5.87 0.17 2.36 1235 20 5.5 1070 20 70/30 14 940example 2 Comparative 5.87 0.17 2.36 1280 20 5.5 1070 20 70/30 14 965example 3

TABLE 2 Peak particle Peak particle Particle Cumulative distributionFrequency size (1) size (2) size 0.62 μm 0.74 μm distribution SSA CDp-iHc p-Br σs Hc (μm) (μm) (μm) (Volume %) (Volume %) 0.28 μm 0.33 μm(m²/g) (g/cm³) (Oe) (G) (emu/g) (Oe) Example 1 1.1 3.4 2.3 14.0 17.6 6.57.6 2.00 3.62 2360 2010 55.99 3033 Example 2 1.1 3.4 2.2 13.8 18.0 6.99.3 1.97 3.63 2400 2010 56.79 3096 Example 3 1.0 3.7 2.3 13.7 18.1 7.09.7 1.93 3.66 2340 2020 56.34 2970 Example 4 1.1 3.0 1.9 15.5 19.9 7.59.5 2.27 3.58 2730 1980 56.89 3335 Example 5 1.1 3.0 1.7 18.2 22.8 8.69.8 2.32 3.60 2640 1980 56.30 3167 Example 6 1.0 3.0 1.7 18.7 23.6 9.612.2 2.41 3.60 2550 1970 55.41 2990 Comparative 1.3 3.5 2.2 11.4 15.73.5 5.5 1.80 3.64 2370 1960 56.31 2943 example 1 Comparative 1.3 3.5 2.310.5 14.2 4.6 6.0 1.71 3.64 2480 1960 56.19 3170 example 2 Comparative1.3 4.1 2.1 10.2 14.6 4.3 6.5 1.68 3.62 2340 2010 55.91 3122 example 3

TABLE 3 F.C. 92.7 mass % + F.C. 93.3 mass % + Nylon 6, 5.7 mass % Nylon6, 5.2 mass % Bonded magnet Bonded magnet Bonded magnet Pellet (1) 4.3kOe orientation (1) 12 kOe orientation (2) Pellet (2) 4.3 kOeorientation (3) MFR Br iHc BHmax Br iHc BHmax MFR Br iHc BHmax (g/10min) (G) (Oe) (MGOe) (G) (Oe) (MGOe) (g/10 min) (G) (Oe) (MGOe) Example1 68.9 3265 1975 2.57 3367 1971 2.77 41.0 3412 1858 2.83 Example 2 83.83294 2034 2.62 3375 1990 2.78 35.5 3392 1878 2.78 Example 3 101.7 33262047 2.68 3380 1985 2.80 50.7 3410 1911 2.83 Example 4 74.2 3281 23412.62 3367 2323 2.79 33.0 3401 2117 2.79 Example 5 73.6 3261 2269 2.583361 2236 2.77 30.2 3396 2056 2.78 Example 6 77.8 3263 2166 2.58 33652143 2.79 34.9 3399 1952 2.79 Comparative 25.3 3118 1952 2.27 — — —Unable to — — — example 1 knead Comparative 24.5 3125 2068 2.29 — — —Unable to — — — example 2 knead Comparative 39.3 3197 1927 2.39 33161890 2.61 Unable to — — — example 3 knead

The invention claimed is:
 1. A ferrite powder for bonded magnets,wherein an average particle size of particles obtained by a dry laserdiffraction measurement is 5 μm or less; a specific surface area is 1.90m²/g or more and less than 2.80 m²/g; a compression density is 3.58g/cm³ or more and less than 3.78 g/cm³, and a compressed molding has acoercive force of 2300 Oe or more and less than 2800 Oe.
 2. The ferritepowder for bonded magnets according to claim 1, wherein a particle sizedistribution curve has two mountains, and out of these two mountains, aparticle size of a smaller particle size peak is 1.2 μm or less.
 3. Theferrite powder for bonded magnets according to claim 1, wherein acumulative distribution value at a particle size of 0.62 μm in acumulative particle size distribution curve, is 13 volume % or more. 4.The ferrite powder for bonded magnets according to claim 1, wherein acumulative distribution value at a particle size of 0.74 μm in acumulative particle size distribution curve, is 17 volume % or more. 5.The ferrite powder for bonded magnets according to claim 1, wherein afrequency of a particle size of 0.28 μm in a frequency distributioncurve, is 6.0 or more.
 6. The ferrite powder for bonded magnetsaccording to claim 1, wherein a frequency of a particle size of 0.33 μmin a frequency distribution curve, is 7.0 or more.
 7. The ferrite powderfor bonded magnets according to claim 1, wherein when kneaded with nylonresin powder to obtain a compound with 92.7 mass % content of theferrite powder, fluidity of the compound is 55 g/10 min or more.
 8. Theferrite powder for bonded magnets according to claim 1, wherein theaverage particle size of the particles is in the range of 1 to 5 μm.